1. Field
The present invention generally pertains to the field of wireless communications, and more particularly to the field of error recovery in wireless communication systems.
2. Background
Over the past two decades, cellular telephones have become increasingly commonplace. During this same period, a number of advances in wireless technology have afforded cellular telephones with more features, better reception, higher bandwidth and increased system capacity. Today's digital and packet-based wireless systems are considerably more advanced than the first digital wireless systems, and show great promise for the future. GSM (Global System for Mobile Communications) was among the first widespread digital wireless systems. GSM was introduced as a second generation (2G) wireless system throughout Europe in the early 1990s and is now operational in over 100 countries worldwide. Over the years the developers of GSM introduced a number of enhancements and improvements, building on the basic voice services of GSM to add various data and speech capabilities to the system. With these improvements GSM has evolved into a system capable of offering a number of enhanced digital mobile voice and data telephony services such as Internet access, multimedia and video.
The GSM enhancements include GPRS, EDGE and GERAN. GPRS, the General Packet Radio Service first introduced in the mid 1990s, is a TDMA wireless packet-based network architecture based on GSM. GPRS is based on the GSM air interface (i.e., the interface between the terminal and the base station) and on the GSM air interface structure of timeslots and TDMA frames. GPRS offers increased bandwidth to users, and more efficient use of bandwidth for operators in as many slots as may be dynamically allocated between voice and data depending upon the demand conditions. This allows a GPRS link to use from one to eight of the slots available per GSM frame, at up to 22.8 kb/s for each time slot. Further, the number of slots for the GPRS up-link and down-link may be allocated independent of each other. GPRS employs four different coding schemes, CS1 through CS4, each of which is a phase modulation coding scheme using Gaussian minimal shift keying (GMSK) modulation. GPRS supports X.25, the low speed packet transmission protocol popular in Europe. GPRS was implemented as a step towards implementing the EDGE system (Enhanced Data for GSM Evolution). EDGE is an enhancement to GPRS which uses the same spectrum allocations as existing GSM systems (e.g. GSM900, GSM1800 and GSM1900). EDGE features nine coding schemes, four employing GMSK modulation and five employing Eight Phase Shift Keying (8PSK) modulation. The four EDGE GMSK coding schemes, MCS1 through MCS4, are akin to the four GPRS coding schemes (i.e., CS1 through CS4). The other five EDGE coding schemes, MCS5 through MCS9, use 8PSK modulation, producing a three-bit word for every change in carrier phase. The use of 8PSK modulation roughly triples the GPRS peak data rates. Another enhancement to GSM, GERAN (GSM Edge Radio Access Network) supports the EDGE network as an alternative radio access network compatible with the 3G GSM-evolved Core Network (CN). The GERAN architecture allows connection to the A, Gb and Iu interfaces of the CN. GERAN is being implemented to deliver packet-based real time wireless services including speech, multimedia, video and Internet access.
Despite the improvements in coding schemes and enhanced features, from time to time, errors occur in wireless systems due to poor reception conditions. To recover from reception errors, EDGE, and the enhancements and services associated with it, provide an incremental redundancy error recovery scheme. When a transmission fails due to the detection of an error, the mobile receiver sends an automatic repeat request (ARQ) back to the base station. In response to the ARQ, the base station transmits the failed transmission using a different encoding scheme. Error recovery is performed by combining the initial message with the second version of the message retransmitted using a different encoding scheme. This conventional system of error recovery increases the likelihood of recovering a failed message, but results in delays due to the ARQ being sent back to the source of the message with a request to retransmit another version encoded differently.